The present subject matter relates generally to nanoscale materials and ways of producing such materials, including semiconductor nanowires.
Nanoscale materials, such as nanoparticles and nanowires, are the subject of considerable research in materials science. Part of the interest stems from the fact that materials formed as nanowires frequently exhibit properties that are dramatically different from the corresponding bulk materials, including unique optical, electronic, and mechanical properties.
Nanowires may be used in a wide variety of applications such as various integrated circuits, chemical and biological sensors, optoelectronic devices, and photovoltaics. For example, there is an interest in developing semiconductor nanowire-based devices for use in various electronic devices and systems to replace or supplement complementary metal-oxide-semiconductor (CMOS) technologies. As another example, nanowires have a large surface area, and thus may be advantageously applied to various sensing modalities and configured as particular sensors, e.g., for biological analytes and other analytes. Reliable large-scale production of nanowires is essential for the commercialization of products based on semiconductor nanowires.
High quality nanowires have uniform diameters along their lengths, and smooth surfaces.[1] Other reported attributes of high quality nanowires include reasonably narrow diameter distributions (standard deviations of 10-20% of the mean diameters), lengths of several micrometers, and straightness of the synthesized nanowires.[2] Nanowires are regarded as low quality if they have kinks and other surface defects, relatively shorter, and branched, especially if these qualities cannot be controlled or arise from synthetic protocols that otherwise should give high quality nanowires.[1, 2] Low synthetic yield, and poor size distributions are also associated with low quality nanowires.[3] Branching in nanowires in itself is usually not regarded as a poor quality if that is the intended morphology, and the branched nanowires can be synthesized in a controlled manner and do not exhibit any other attribute of low quality nanowires. [3, 4]
Semiconductor nanowires may be synthesized using a range of conditions, from high temperature (e.g., less than 1100° C.) gas-phase reactions,[5] to relatively low temperature (e.g., less than 250° C.) solution-phase conditions. Solution-phase routes to semiconductor nanowires are of particular interest due to the potential for size and shape control, chemical surface passivation, colloidal dispersibility, and adaptability to high throughput continuous processes. Solution methods allow greater control over structure and function than gas phase methods.[6]
Solution-phase routes to semiconductor nanowires include non-catalyzed (e.g., oriented attachment, and solvothermal/hydrothermal growth), and catalyzed (e.g., supercritical fluid-liquid-solid growth, and solution-liquid-solid growth) approaches. The most reliable and reproducible method has proved to be the catalyzed methods. In the catalyzed approach, small metal droplets (also called metal seeds, metal seed particles, or catalyst) are used to induce the asymmetric crystallization of semiconductors from precursors in solution.[7] Among the catalyzed solution-phase routes, solution-liquid-solid approach affords the use of lower temperatures (e.g., less than 250° C.) to grow semiconductor nanowires with such advantages as nanowire crystallinity, length, and diameter control.[8] The solution-liquid-solid (SLS) is so named based on a proposed mechanism wherein the nanomaterials precursors are in solution, then partition in to the liquid catalyst, and lastly the solid nanowire is grown.
Common among the solution-phase routes to many nanomaterials is a hot-injection technique in which a precursor solution at room temperature is injected into a second precursor solution held at a certain elevated temperature in order to rapidly produce a large amount of monomers to trigger a burst of nucleation and subsequent growth of nanomaterials in a controlled manner in the reaction system.[9] The solution-phase routes are typically performed as a small scale batch process to achieve the required control over thermal and mass transport properties needed to control nucleation and growth. However, even when the hot-injection reaction is done on the small scale, the control of the needed parameters is not as precise as desired.[10, 11]
Conventionally, cadmium selenide nanowires are produced in solution by the hot-injection method, where the reagent solution containing the catalyst and selenium source is injected into a hot (220° C.-300° C.) vessel containing the cadmium precursor solution. The hot injection method is also widespread in the synthesis of other semiconductor nanostructures like CdS, CdTe, PbS, PbSe, ZnSe, etc.[3, 12-15] It is widely held that the rapid temperature jump experienced by the injected reagent solution is necessary to nucleate the metal seed particles, and induce rapid growth of the nanowires from the seeds.[7, 9] Generally in the growth of semiconductor nanocrystals, the requisite supersaturation and subsequent nucleation can be triggered by rapid injection of a precursor solution into a vigorously stirred flask solution containing a hot (150-350° C.) solvent or another precursor solution.[15] The growth of nanowires typically takes 1-5 minutes, and the temperature must remain constantly high post-injection, as a drop in temperature can lead to a variety of other nanomaterial morphologies such as rings, quantum dots, and other undesired by-products.[16]
Similarly, nanowire production may suffer if all the reagents are mixed together and concurrently heated to the reaction temperature. In the batch process, where heat transfer is very slow due to the volume of reagent being heated, it is difficult to rapidly increase the precursor concentration above the nucleation threshold, which leads to the short nucleation critical to form nanoparticles.[15] In the hot injection method, this nucleation burst arises from the rapid injection of new reagents and the dramatic alteration of the temperature profile.[17] The poor production of nanowires may be due to the numerous side reactions that may occur at lower temperatures that serve to consume all the reagents. A side reaction of particular note is the formation of magic sized clusters, which can form at relatively low temperatures, including room temperature.[18-21] Magic clusters are small clusters of a few to several hundred molecules, which are unusually stable compared to other clusters of similar sizes. Not only do these clusters consume the precursors and decrease the yield of the desired product, but they may also be intermediates in the synthesis of certain morphologies of nanomaterials, and produce undesired by-products.[20] Thus, despite the difficulties of the hot injection process, it is the conventional route to the solution synthesis of nanowires.[22, 23]
A disadvantage of the hot-injection method is that it is not scalable. For example, to scale up the production of nanowires in a batch hot-injection process, large amounts of cool reagents solutions must be injected into large amounts of hot reagent solutions. Here, the temperature and the reaction profiles may be different and these parameters may affect the quality of the produced nanowires. Additionally, further scale up or continuous production may be hampered because the set-up must be constantly disassembled after each process, and the process may need to be optimized at each scale. Quantum dots were originally made by hot-injection, but the commercial process for production of quantum dots is a continuous flow method.[24] A variety of methods and apparatuses for making quantum dots via flow technology have been patented.[25-27]
While quantum dots and nanowires are both made by the hot injection method, and in some case have extremely similar reagents used, innovations made for quantum dot synthesis that enabled translation of the quantum dot synthetic process to continuous flow methods have not been successfully applied to nanowire synthesis.
For example, trioctylphosphine oxide (TOPO) is a chemical used as a solvent in the hot-injection synthesis of both CdSe nanowires [4, 6, 8] and quantum dots [15, 28]. Traditionally, in the example of CdSe nanowires, solution-based synthesis of CdSe nanowires (NWs) have been carried out using trioctylphosphine oxide (TOPO) as the solvent in a batch synthesis procedure. Even highly pure TOPO contains impurities that have a strong impact on the reliability of nanomaterials synthesis.[1, 2, 29] Despite the problems with TOPO, the recommended procedure in nanowires synthesis is to purify TOPO in-house, and dope in trace amounts of alkylphosphinic acids to produce the desired morphology.[2] These same issues impact quantum dot synthesis, however, the quantum dot synthesis has been optimized to reduce or eliminate TOPO. This not only improves the quality and reliability of quantum dot synthesis, but is an enabling factor that allows quantum dots to be produced in a flow reactor.
TOPO limits the translation of many syntheses to a flow configuration because it is solid at room temperature, and thus problematic to flow.[30] It has been shown that TOPO can be eliminated[31-37] or reduced/diluted[38, 39] in quantum dot synthesis without impacting the quality of the quantum dots. This then enables a flowable reagent mixture, which enables translation of the system to a flow reactor. Elimination or reduction/dilution of TOPO in CdSe nanowire synthesis has not been reported.
Similarly, the conventional selenium source for the synthesis of metal selenide nanowires is selenium powder dissolved in trioctylphosphine (TOP-Se).[6, 14, 16] A similar procedure is used to create other chalcogenide sources (E=S, Se, Te) such as TOP-S,[40] TOP-Te,[8] etc. However, TOP-Se is very air-sensitive and may result in reproducibility problems with the formation of the nanowires, owing to the impurities found in trioctylphosphine.[41] As a result, alternatives to TOP-E in solution phase metal-chalcogenide nanomaterials synthesis are needed. Again, the synthesis of quantum dots has been optimized to use alternate chalcogenide sources, such as elemental selenium dissolved in 1-octadecene (Se-ODE),[42-46] or S-ODE,[45, 47]. However, again, application of metal-chalcogenide semiconductor nanowire synthesis using E-ODE as the selenium precursor has not been reported.
The batch hot injection method is usually accompanied by a temperature drop when the cool reagent solution is added to a hot solution to initiate the process of nanowire formation. After the injection, the reaction temperature is maintained while the nanowires are allowed to grow for a specific amount of time, usually 1-5 min. The temperature drop, as well as the time it takes the temperature of the hot reagent solution to return to the set temperature, scales with the amount of the injected cooler reagent solution. Further, Se-ODE is usually prepared at a concentration that is an order of magnitude less concentrated than TOP-Se and as such requires a substantially larger volume of the solution for the same number of moles of selenium. Typically, TOP-Se is prepared at a 1M concentration, while Se-ODE is prepared at a 0.1M concentration. The large volume of Se-ODE that is needed for the same number of moles as in TOP-Se will result in a large temperature drop when the Se-ODE solution is used is the batch hot injection process. Such a large temperature drop is problematic for any nanowire synthesis using the hot injection method.
Three examples of nanowire morphologies grown via a flow process have been reported that all use a similar apparatus. Importantly, none use a continuous flow process, which is a limitation of the apparatus and method design. Essentially, all three conventional methods contain a substrate with attached catalysts. Reagents are flowed across the surface, and nanowires grow out from the surface. The reaction is halted, and the substrate is removed from the reactor and the nanowires are harvested from the substrate.
For example, a conventional method using a flow reactor prepared gold nanoparticles and deposits them on a Si wafer. Each substrate was 5×20 mm, and the flow cell could hold one substrate.[48] The flow technology involved flowing supercritical fluids containing either reagents or solvent over the substrate for some time, then quenching the process and removing the substrate to examine the nanowires. This method also requires specialized chemicals that have the desired properties in the supercritical fluid, and is not generally applicable. A heating block could hold up to 6 reactors, providing some demonstration of scalability. However, the wafer size itself probably is not scalable, as too large a wafer can lead to gradients in chemical precursor concentration as the wires growing on front portion of the wafer consume the reagents and “starve” the wire growth at the back portion of the cell.[49]
Zinc Oxide nanowires were grown using the chemical bath deposition method using a similar apparatus, but using flowing liquid phase, rather than super critical fluid phase. In this example, the catalyst-coated substrate was actually incorporated as one of the walls of the reactor.[49, 50] Again, the reactor would need to be disassembled to remove the product, and the reactor could only be run while catalyst remained on the wall.
Similarly, using a catalyst coated substrate wall was recently reported by the group of Hollingsworth.[51] In this example, a 10×10 mm2 squares of bismuth were coated onto a 15×25 mm2 Si wafer using electron-beam deposition. The substrate was then assembled into the reactor using a gasket system to attach the substrate to a microfluidic chip. Precursor solutions were then flowed over the heated bismuth substrate. When the reaction was over, the reactor was disassembled and the nanowires on the substrate were removed. Notably, the authors report problems with the solidification of their precursors. In the example, a Cd(TDPA)2 precursor is created using the standard TOPO based preparations. The Cadmium precursor was then diluted with a large volume of TOP, which was reported to prolong the time before the solution solidified, and aided in the flowability. However, this did not prevent solidification, and the solutions had to be heated to ˜100° C. to melt them, before loading them in the pumps to flow into the reactor. The authors also report that the feedlines were sometimes heated with a heat gun to prevent solidification of the precursor solutions. The nanowires produced were typically quite short, typically under 1 micron, even at very long reaction times (30 minutes). The nanowires also had a tapered morphology, with one end substantially narrower than the other, and coming to a point.
An apparatus and methods are needed to synthesize high quality nanowires in a flow reaction in larger quantities and results in reproducible, high quality, nanowires.